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D2786-91 – Standard Test Method Technical Guide
📐 Scope and Sample Requirements
This test method, ASTM D2786‑91 (Reapproved 2016), specifies the determination of seven saturated hydrocarbon types and one aromatic type in gas‑oil saturates fractions by high ionizing voltage mass spectrometry. It applies to saturate petroleum fractions with average carbon numbers from 16 to 32. The saturate types include alkanes (0‑rings), single‑ring naphthenes, and fused naphthenes with 2, 3, 4, 5, and 6 rings. The nonsaturate type is monoaromatic. Noncondensed naphthenes are analyzed as single rings. Samples must be nonolefinic and contain less than 5 % by volume monoaromatic. Composition data are reported in volume percent.
The values stated in SI units are the standard; values in parentheses are provided for information only. Users must establish appropriate safety and health practices before use.
⚙️ Test Procedure and Mass Spectral Analysis
The relative abundance of each hydrocarbon type is obtained by mass spectrometry using a summation of mass fragment groups most characteristic of each molecular type. Calculations are performed with inverted matrices that are derived from ion intensity calibration sensitivities and are specific for the average carbon number of the sample. The procedure requires a high ionizing voltage source and is calibrated with known standards or reference data. Referenced ASTM methods include D2549 for fraction separation and D3239 for aromatic types analysis.
The characteristic mass groupings used in the analysis are defined in Section 3.1.1 of the standard and are summarized below.
📊 Characteristic Mass Groupings and Key Data
| 🟦 Hydrocarbon Type | 📏 Characteristic Mass Summation | 🔢 Ring Count |
|---|---|---|
| Alkanes | Σ71 = 71 + 85 + 99 + 113 | 0 |
| 1‑Ring Naphthenes | Σ69 = 69 + 83 + 97 + 111 + 125 + 139 | 1 |
| 2‑Ring Naphthenes | Σ109 = 109 + 123 + 137 + 151 + 165 + 179 + 193 | 2 |
| 3‑Ring Naphthenes | Σ149 = 149 + 163 + 177 + 191 + 205 + 219 + 233 + 247 | 3 |
| 4‑Ring Naphthenes | Σ189 = 189 + 203 + 217 + 231 + 245 + 259 + 273 + 287 + 301 | 4 |
| 5‑Ring Naphthenes | Σ229 = 229 + 243 + 257 + 271 + 285 + 299 + 313 + 327 + 341 + 355 | 5 |
| 6‑Ring Naphthenes | Σ269 = 269 + 283 + 297 + 311 + 325 + 339 + 353 + 367 + 381 + 395 + 409 | 6 |
| Monoaromatic | Σ91 = 91 + 105 + 117 + 131 + 145 + 159 + 173 + 187 + 201 + 215 + 229 + 243 + 257 + 271 | 1 (aromatic ring) |
⚠️ Critical Note: The method is valid only for nonolefinic samples with less than 5 % volume monoaromatic. Exceeding these limits degrades accuracy. Prior separation using ASTM D2549 is recommended when aromatic content is high.
❓ Frequently Asked Questions
🔍 What is the role of the characteristic mass groupings?
Each grouping (e.g., Σ71 for alkanes, Σ69 for 1‑ring naphthenes) sums the ion intensities of mass fragments that are uniquely associated with a specific hydrocarbon type. These groupings are the foundation for the matrix calculation that converts raw spectral data into volume percent composition.
💡 How are noncondensed naphthenes handled?
Noncondensed naphthenes are treated as single‑ring naphthenes in this method. This simplification avoids ambiguity in ring counting and aligns with the characteristic fragmentation patterns observed for these structures.
⚡ What is the function of the inverted matrices?
The inverted matrices are derived from ion intensity calibration sensitivities and are specific to the average carbon number of the sample. They mathematically deconvolute overlapping spectral contributions, enabling accurate quantification of each hydrocarbon type from the measured mass groupings.
📌 Why is the monoaromatic limit set at 5 %?
The matrix calibration and sensitivity factors were developed for samples with low monoaromatic content. Above 5 % volume, interference from aromatic fragment ions increases significantly, compromising the accuracy of saturate‑type assignments. Users should verify compliance with this limit before analysis.